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Phytoremediation of Contaminated Groundwater

Ray Douglas:
Groundwater is one of our most important natural resources. It provides
drinking water to more than half the population in the United States
and even a greater percentage of the people in the developing world. So
when it becomes contaminated, we have to do our best to find
cost-effective ways to restore that contaminated groundwater to
conditions not only that we can use now but for future generations.
Phytoremediation happens to be one of the options that can be used to
meet that goal.

Phytoremediation: what is it and how is it connected to the
future of our groundwater?

[Music]

Ray Douglas: I'm Ray
Douglas, and this is CoreCast, your science podcast for a changing
world.

Jim Landmeyer is a USGS research hydrologist for the South
Carolina Water Science Center. Jim, thanks for joining us today.

01:04

Jim Landmeyer: You're
welcome.

Ray Douglas: Before we
sat down today, I Googled "define phytoremediation" and the number one
link led me to the USGS Toxic Substance Hydrology Program, and at the
top of the page was your definition of 'phytoremediation'. For
starters, could you tell us just what is phytoremediation?

Jim Landmeyer:
Phytoremediation is the use of living plants in the way that they
interact with the hydrologic system to either extract contaminated
groundwater from the water table or to decrease recharge to the water
table through often contaminated, unsaturated zone sediments. Once
these contaminants are into a tree, the tree itself can either
volatilize the contaminants to the atmosphere or the tree can degrade
the contaminants into harmless compounds inside the tree tissue itself.

Ray Douglas: How long
have we known about phytoremediation?

Jim Landmeyer: The
term 'phytoremediation', and some folks actually like the term
'phytotechnology', in any case, the term itself probably first was used
in the early 1990s.

02:09

However, the processes for which phytoremediation is the
umbrella term for actually had been used by society for generations. In
many cases overseas, for instance in Europe, the term 'bog wastes' or
'wetlands' were places obviously containing plants, places where waste
were thrown to be remediated, to be cleaned up.

There has been many instances where land farming has been used
where wastes are applied to areas that are then planted for the plants
to handle the wastes. Those aren't termed phytoremediation but they
helped establish the fundamentals for phytoremediation.

Ray Douglas: Jim, it
seems to me that there would be some limitations as to which sites
could actually use phytoremediation. Aren't contaminants sometimes deep
below the earth surface?

Jim Landmeyer: This is
true because groundwater can be anywhere from near land surface to as
much as 200 to 300 feet below land surface. Most trees, their roots
tend to be within the upper few feet of soil
because those trees tend to take up rain water as their source of water.

03:12

Ray Douglas: So what
depth range is best suited for phytoremediation?

Jim Landmeyer: For a
phytoremediation of contaminated groundwater to be the most effective,
the depth to water table from land surface should be no greater than
about 15 to 20 feet below land surface, because any depth greater than
that would probably be a less efficient uptake of groundwater.

Ray Douglas: How much
uptake of groundwater are we actually talking about?

Jim Landmeyer: Some
large trees, tens of years old, say, a poplar tree, can pump tens of
gallons of water, some of that groundwater, through their systems on a
daily basis. So if you can imagine, if you have a site where 3,000
poplar trees are planted, each pumping each 10 gallons of contaminated
groundwater per day, the effect on the water table can be impressive.

04:04

Ray Douglas: What are
some of the types of groundwater contaminants that can successfully be
removed from the water table using phytoremediation?

Jim Landmeyer: Types
of contaminants that can be taken up by trees and be phytoremediated
include gasoline compounds like benzene, toluene, creosote compounds
like naphthalene, chlorinated solvent compounds like perchloroethylene
or tetrachloroethylene or TCE. There are various other compounds that
can be taken up by plants through phytoremediation such as explosive
compounds all the way to including inorganic such as metals.

Ray Douglas: That's
quite a list of contaminants. But how is it that the tree can actually
survive the uptake of all those compounds?

Jim Landmeyer: Well,
as the old saying goes, "It's not the poison that kills you, it's the
dose of the poison," and it sort of holds true for phytoremediation of
contaminated groundwater. For instance, if you take a plant, be it a
young plant or a mature plant, and you put it in a vat of gasoline or a
vat of dry-cleaning fluid, it will definitely not survive.

05:08

On the other hand, if you put the same type of plant in an
area where there are much lower concentrations that are dissolved in
water, yes, those plants can not only survive but they will take up
those contaminants.

And trees that have been around and have evolved starting
about 400 million years ago have developed quite effective ways to deal
with threats to their existence. So trees do have the natural ability
to take up contaminants that they see both natural and then therefore
man-made and have enzyme processes in place that permit those
contaminants to be handled by the plant where they turn it into
something either innocuous and not harmful or they actually
compartmentalize that harmful compound into an area that won't affect
their systems.

This process is very similar to how we as humans in our liver
detoxify chemicals. In fact, the term 'green liver' has been given to
the ability of plants to process and to detoxify these chemicals that
they encounter.

06:08

Ray Douglas: So I
guess we are just pushing the evolution of trees along by introducing
phytoremediation.

Jim Landmeyer: Well,
in one way, we are. People are actually, and this is something that we
haven't specifically done in the USGS, but people are actually taking,
for instance, bacterial enzymatic capabilities to grade certain
contaminants that plants can't degrade now and through genetic transfer
actually transferring from the bacteria to the tree the ability for a
tree to now degrade at compound.

Ray Douglas: What key
difference is there in this process of phytoremediation of contaminated
groundwater and older methods of remediation?

Jim Landmeyer: Well,
there are many different ways to remediate contaminated groundwater.
The common denominator between many of the more engineered methods is,
in order to help remediate, you have to add a lot of energy into that
system that you're remediating, the energy in terms of a physical
capital investment. Adding wells, injecting slurries, pumping wells,
trapping contaminants, all those remediation technologies, the
traditional engineering technologies, take a great input of energy,
which is an input of funds.

07:17

With phytoremediation, the basis for the energy input, other
than actually putting the trees in the ground and maybe pruning them
from time to time, is actually taken care of by solar radiation.

Ray Douglas: So would
you say that phytoremediation is a more cost-effective way to clean up
contaminated groundwater?

Jim Landmeyer:
Phytoremediation can be more cost-effective than your traditional,
classic engineering systems such as a pump-and-treat or vapor
extraction recovery system or even oxidant-injected systems.

Ray Douglas: What is
it about phytoremediation, the trees, the process, that captured your
attention to do this research?

Jim Landmeyer: The
interaction with trees, with subsurface sources of water like
groundwater, at first blush may not be apparent, because after all
roots are hidden below ground out of sight, and groundwater is hidden
below ground out of sight. So we tend to think that trees get their
water primarily from rain.

08:19

In many cases, whether a tree is getting its water from
rainfall or from rain that's infiltrated and recharged in aquifer, the
tree itself and forests of trees in a given area tend to remove and
send back to the atmosphere in some places up to 70% of the water that
entered that area. So trees are a huge, important component of
processing water through the hydrologic cycle.

Ray Douglas: So I
guess now we need to begin thinking of trees as groundwater pumps.

Jim Landmeyer: Yes and
no, Ray. Trees do move water from one compartment, the subsurface, to
the other, the air, much like a pump would, say, remove water from the
ground and put it into your house. But with a pump, you are actually
adding energy to turn a lifting mechanism that moves out water with
plants.

09:15

The only energy that they require is the solar energy from the
sun that keeps them alive, but also the difference in wetness between
the ground, which tends to be very wet versus the air which can be dry.
That's what drives evaporation, and that's what drives the passive
movement of water through a tree.

Ray Douglas: I
understand that you have a new textbook in the works. Could you tell us
a little about that?

Jim Landmeyer: Yes.
We've been given the opportunity to compile and prepare and publish a
textbook called "The Introduction to Phytoremediation of Contaminated
Groundwater" and it will be available sometime in 2011. This is the
type of book that I wish was available when I started my first
phytoremediation project back in 1998.

10:09

Ray Douglas: Jim,
thanks for taking time out of your busy schedule to talk with us today.
Any closing thoughts on phytoremediation and its future?

Jim Landmeyer: You're
welcome. This is an important issue.

Hopefully, someday we'll simply be able to go to a
contaminated site, plant trees and grasses, and then walk away knowing
that the processes of phytoremediation are occurring and that the
groundwater system is being cleaned up. We're not there yet. Right now
we still need to collect good reproducible data that shows that these
phytoremediation processes are hearing the fundamental principles in
physical laws.

Every part of the world has a certain allocation of solar
energy, that you can actually measure in kilowatts, that is being input
to a system. You can take advantage of that through using
phytoremediation to drive the remediation of the system by pumping
groundwater, taking advantage of the natural interaction that plants
need to have to move water through their system. That is green
technology at its best.

11:13

[Music]

Ray Douglas: We'd been
talking with Jim Landmeyer, a USGS research hydrologist in the South
Carolina Water Science Center.

CoreCast is a product of the U.S. Geological Survey, U.S.
Department of the Interior. Thank you for listening.